Posted
by
samzenpus
on Tuesday February 16, 2010 @03:04PM
from the speed-of-the-sound-of-loneliness dept.

Cytotoxic writes "What to do with all of those leftover Valentine's Day chocolates? — a common problem for the Slashdot crowd. The folks over at Wired magazine have an answer for you in a nice article showing how to measure the speed of light with a microwave and some chocolate. A simple yet surprisingly accurate method that can be used to introduce the scientific method to children and others in need of a scientific education."

The demonstration works because microwave ovens produce standing waves -- waves that move "up" and "down" in place, instead of rolling forward like waves in the ocean.

Ocean waves don't "move forward".

While the individual particles do not, the wave itself does.

He is mistaken on the meaning of a standing wave. It is not the same as a transverse wave (which seems to be why he is comparing them to a longitudinal wave).

The oven is designed to be just the right size to cause the microwaves to reflect off the walls so that the peaks and valleys line up perfectly, creating "hot spots" (actually, lines of heat).

Disproved by direct observation. Go into any store and you'll see microwaves in various sizes. The perfect microwave doesn't have "hot spots".

Again, he's wrong about it being 'designed' for the purpose of having hot spots, but the design does result in hot spots. These occur regardless of oven size, they will simply be located in different locations. This is caused by reflections off internal surfaces acting like two signal sources.

While it may be conceivable to create a 'perfect' microwave with no standing wave nodes, it would be pointless. Besides, he wasn't using an 'ideal' microwave, just a regular off-the-shelf microwave, which does have standing waves.

False. Disproved by direct observation. Stand on a pier sometime and look at the waves. The spot where a maximal low is does not become the maximal high; instead a spot in front of it is the maximal high. There is, indeed, a rolling forward effect.

Disproved by direct observation. Go into any store and you'll see microwaves in various sizes. The perfect microwave doesn't have "hot spots".

Just because there are microwaves of different size does not mean that none of them

alas, if only most slashdotters were in Japan. There the women or girls give low cost chocolate to all the guys at work or in school class. It's called "obligation chocolate" or "pity chocolate", but if there is a guy they really like they give expensive or homemade chocolate with a gift "prospective winner chocolate".

actually, the top of line is considered to be home-made chocolate, which can be made by cheating a bit with cheap candy bar, melted and a couple ingredients added to kick up a notch. So hand effort makes the supreme valentines day gift, within reach of the poor for their special someone.

The brain makes assumptions while reading, speed implicates something that we imagine moving by itself unless we talk about speed of growth, as it is the case for fat. That, along with the "digestion" topic GP, could explain this mistake from me.

Actually, using expression like "approximately" or specifying an error margin instead of using terms like "exact" like the GGP did is a pretty scientific standard and it is overall a good habit in all sphere of life.

In general, one should be doubtful about "absolute truths", "exact calculations", "100% efficiency" or the like;-))

This experiment has lots of problems. *nom nom nom* First, microwave ovens don't always precisely match the given frequency. *chomp chomp* Second, and more importantly -- *chew chew swallow* -- identifying the hotspots and measuring the distance between them is difficult and error prone. *nom nom* And that's even when the chocolate is fresh! It's worse after it's already been partially melted. *stuff face* So I had to perform many experiments, using fresh chocolate each time, to get an accurate measurement.

More problems than that, even. The article does nothing to address the puzzled questions that my son (or even my wife, who is smart but no techie) would ask if I showed them this. That's where the REAL lessons are:

1 - "How does this measure the speed of light when we are using the microwave and not a flashlight?" (Answer: because microwaves and visible light are both forms of electromagnetic radiation... so is infrared, what you feel on your face when you stand by the campfire, and radio waves that bring music to our car stereos.)

2 - "Why does this experiment mean anything about speed? We are measuring a distance, not a speed." (Answer: because the wavelength is related to frequency by the speed of propagation. Think about shaking one end of the rope and watching the waves travel down it. Frequency is how many times per minute you shake. Each shake makes a peak and the space between peaks is how far the previous peak moved down the rope before the next shake. That's how wavelength and frequency are related by propagation velocity.)

If your child is still paying attention at the end of that thought experiment, you know he's a scientist. Buy her a model rocket or a microscope. If not, give her a set of watercolors or a video camera.

If your child just eats the chocolate and asks for more, then just buy him a guitar.

You seem to be confusing a complete lesson on electromagnetic radiation with an experiment to determine the speed of light. An experiment only does the one thing, and the instructions are just for how to perform the experiment. (It is lacking in theoretical hints, but does tell you to look into standing waves.)

An experiment is worthless if you don't know enough of the theory to interpret the results. Without that knowledge it's not an experiment to determine the speed of light, but just an experiment to determine the wavelength of the microwave.

You also have to know the frequency for this to work, which involves trusting either TFA or your microwave manual, which makes it much less interesting as a pure science experiment. But still delicious.

Red Symons [wikipedia.org] has a Bachelor of Science degree (studied pure mathematics and computer science) but is perhaps best known for his role as lead guitarist for Aussie glam rock band Skyhooks [wikipedia.org], as well as as a tv / radio presenter with an acerbic wit.

This experiment has lots of problems. *nom nom nom* First, microwave ovens don't always precisely match the given frequency. *chomp chomp* Second, and more importantly -- *chew chew swallow* -- identifying the hotspots and measuring the distance between them is difficult and error prone. *nom nom* And that's even when the chocolate is fresh! It's worse after it's already been partially melted. *stuff face* So I had to perform many experiments, using fresh chocolate each time, to get an accurate measurement.

In conclusion, this experiment rules. *nom nom nom nom*

Yes, that's why there is such thing as uncertainty. If she propagated the uncertainty both in the frequency and the "eye-balling" part correctly it will probably include a large enough error bar pass the actual speed of light, making this an accurate, but imprecise experiment.
Oh wait, are we talking about a fun project to be doing with you kid, or a collage physics lab.

I wonder if this works with that nasty fake stuff that Palmer makes. You know the stuff, made with vegetable oil instead of cocoa butter, and labelled "Milk Chocolate flavored candy"? It may change the melting dynamics just enough to invalidate the whole experiment. I bet that stuff isn't even good enough for science experiments.

1. You aren't running the risk of burning perfectly good chocolate to perform the experiment.2. You are more likely to have leftovers of Palmer's, assuming you had some in the house in the first place, since no one in their right mind would actually eat it.

Remember, even Twinkies are good enough for science experiments. It's simply a matter of choosing the right experiment.

It's stated on the microwave (or in its documentation). They don't get that by knowing the speed of light, but rather by the construction of the device. You could build your own microwave generator to be sure, but assuming that the manufacturer is giving you the correct value is good for an estimate.

Well, for the purposes of verifying the speed of light without circularity, (in principle) you could construct a high-speed clock divider to divide the microwave signal down to a frequency you can measure using a pocket watch. Obviously whoever constructed the microwave knew the speed of light, but you don't need to assume it to make this measurement.

That is normally listed on the back of the microwave, and is almost always in the 2.4GHz range. You also don't need the figure that precise, given that your result is basically likely to have only about 1 sig fig anyhow. That's fine, since light is extremely close to 300,000,000 m/s. You need 4 sig figs before a discrepancy starts to show up, and you aren't getting that out of an experiment like this.

Nobody is claiming this is USEFUL, like you are finding out something amazing. After all, we know the speed

While I have some Valentines chocolate remaining uneaten, none of it is 'left over'.

It's far too valuable to melt (thus destroying many of it's finer qualities), a) because it's quality chocolate rather than crap picked up at the convenience store, and b) my wife handpicked the assortment for me catering to my tastes.

Have someone throw the microwave at you so fast that the blue shift turns the microwaves into visible light. By observing the colour, and using the speed of the microwave, you can calculate the frequency.

It's a neat trick, albeit an old one. But it's not quite a real measurement of C. The problem is that you're given the frequency to start with, and a smart high school student will tell you that means you also know the wavelength. So if you trust the frequency rating of the microwave then the only thing you're really doing is verifying that the ruler you're using is accurate.

"Valentine's Day chocolates? a common problem for the Slashdot crowd."

What is this Valentines Day? What does it have to with chocolate? ?

I think your new here, as that not on the list of IT holidays...

And there is NEVER left over chocolate, err.. REAL chocolate.. not that crap Hershey stuff (AND NO I don't buy Cadbury in the US! I know its licensed to be made by Hershey.. I have importers bring in the REAL CADBURYS!)

Just like beer if its made in the US, its junk. Same goes for chocolate.

Good lord. Out of all the chocolate you could import, you choose Cadbury? That's worse than importing Budweiser. 57% sugar is not what you call real chocolate. Seriously, their cocoa content is the bare minimum they can legally get away with to still call it "chocolate". And chocolate shouldn't be adulterated with vegetable fat.I know Brits feel a certain national pride for their last mass-market chocolate brand, but yuck.

Actually, it is measuring the wavelength of the microwave radiation, and assuming the stated frequency is correct, calculating the speed of light from that. However, this is circular, sense the frequency was most likely also calculated by measuring the wavelength!

Just give way to your scientific curiosity... and quite literally at the speed of light, this will be the last "distance between peaks and valley, lining up perfectly" that you'll ever get to measure.;-)
All further computations shall be done by judges and attorneys, but you'll have lots of time to spare for writing great code afterwards.;-}
BTW, the safety instructions miss the warning:

Remove all items such as knives, forks and frying pans she might feel an impulse to stab or hit you in the head with..."